Cyclin-dependent kinase 5 (Cdk5) is a member of the family of cdks. Unlike other cdks, Cdk5 activity is detected mainly in postmitotic neurons. Association of Cdk5 with a neuron-specific regulatory subunit, either p35 or its isoforms p39, is critical for kinase activity. We have analyzed Cdk5 roles using conventional knockout mice of Cdk5 (Cdk5-/-). Cdk5-/- mice exhibit embryonic lethality associated with disruption of the cortical laminar structures due to defective neuronal migration. Additionally, chromatolytic changes such as a ballooned cell soma with eccentric nucleus were observed in neurons of Cdk5-/- mice. This is accompanied by an accumulation of phosphorylated neurofilament-H (pNF-H) immunoreactivity. This accumulation of pNF-H was typical seen in the cell soma of the motor neurons in the cranial nerve nuclei and spinal cord ventral neurons. Addition to the important roles of Cdk5 in developmental stage, it is also implicated that Cdk5 plays multiple functions in mature CNS such as phosphorylation of neurocytoskeletons, synaptic transmission and dopaminergic signaling . To determine the role of Cdk5 in adult CNS, we have begun to analyze the mice for conditional disruption of Cdk5 gene using Cre-loxP system, with controlling spatial and temporal gene disruption (Cdk5D/D). These Cdk5D/D mice were viable and fertile and showed slow progression of locomotor abnormalities including hunched back posture. Cdk5-/- mice exhibit embryonic lethality associated with disruption of the cortical laminar structures due to defective neuronal migrations. Whereas p35-/- mice showed milder phenotypes than Cdk5-/- mice due to the redundancy of Cdk5 activator isoforms. Moreover p35-/-p39-/- mice with the phenotype identical to Cdk5-/- mice confirm redundancy in in these isofroms. Neuronal birthdate labeling by BrdU revealed an inverted layer structure in cerebral cortex of Cdk5-/- mice. An inverted pattern of layer structure in the cerebral cortex is a well-known characteristic of the reeler and scrambler/yotari. These mutant mice exhibit nearly identical phenotypes suggesting that the gene products mutated in these mutants, Reelin and Dab1 respectively, act in a common signaling pathway during cortical development. While Cdk5-/- and p35-/- demonstrate some similarities with reeler and scrambler/yotari mice, the development of the embryonic cerebral cortex in Cdk5-/- and p35-/- mice also shows significant differences from reeler and scrambler/yotari such as the split of the preplate. In the wild-type mice, successive waves of migrating neurons form the cortical plate in an inside-out fashion, splitting the preplate into the marginal zone and subplate. In reeler and scrambler/yotari mutants, the migrating cortical neurons appear incapable of splitting the preplate, and cortical plate neurons stack up in the inverted order beneath the preplate. In Cdk5-/- and p35-/- mice, earlier-born neurons successfully split the preplate, however, late-born neurons stack up in an inverted layer under the subplate. Two general modes of neuronal migration have been described in the developing nervous system: nuclear (or somal) translocation (also called nucleokinesis) and locomotion. Presence of these two modes of radial migration in the cerebral cortex is recently observed using living slice culture. Based on our observation in the cerebral cortex of the Cdk5-/- mice, we proposed that earlier-born neurons might use nuclear translocation mode which is Cdk5-independent whereas migration mode of late-born neurons is Cdk5-dependent in the cerebral cortex . Recently, developmental defects of brain stem structures have been reported in Cdk5-/- and p35-/-p39-/- mice including the lack of inferior olive, however, characterization of migration defects of these abnormalities remains to be elucidated. In order to characerize Cdk5-dependent and independent modes of neuronal migrations, we analyzed neuronal migrations in the hindbrain of Cdk5-/- mice in detail. Selective defects of neuronal migration were identified in facial nucleus and inferior olive, and rest of other brainstem nuclei formed normally including pontine nucleus which is considered to be formed by nuclear translocation mode of migration. Since neuronal migration defects of facial nucleus and inferior olive have been described in reeler mice, a possible relation of Cdk5/p35 with Reelin signaling and its effect on neuronal migration were further studied using double mutant mice for p35 and Dab1 as well as Dab1 mutant mice. The microtubule-associated protein tau is a developmentally regulated family of neuronal phosphoproteins. The increase in tau phosphorylation reduces its ability to bind and stabilize axonal microtubules, allowing microtubule rearrangements underlying axonal growth. The activity oCdk5 is tightly regulated by association with its neuronal activators, p35 and p39. Although tau can be phosphorylated by Cdk5 in vitro, the in vivo roles remained to be unclear. We have demonstrated that tau is phosphorylated by Cdk5/p39 during brain development, resulting in a reduction of its affinity for microtubules. The p39 expression level was higher in embryonic hindbrain and spinal cord and in postnatal cerebral cortex while that of p35 was most prominent in cerebral cortex throughout brain development. The ability of Cdk5 for tau phosphorylation was higher in association with p39 rather than in association with p35. Tau phosphorylation at Ser-202 and Thr-205 was decreased in Cdk5-/- mice brains but not in p35-/- mice brains, suggesting that Cdk5/p39 is responsible for in vivo phosphorylation of tau. This suggests that Cdk5 may provide the microtubules with more dynamic property in a region-specific and developmentally regulated manner through the tau phosphorylation which would be necessary for a proper brain development. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by selective loss of motor neurons in the brain and spinal cord. Neurotoxicity mediated by glutamate is thought to play a role in t he neuronal death through intrcellular calcium-dependent signaling cascades. Cdk5 has been proposed as one of the calcium-dependent mediators that mmay caiuse neuronal death observed in this death. Cdk5 is activated in neurons by the association with its activators, p35 or p39. The calcium activated protease calpain cleaves p35 to its truncated product, p25, which eventually causes the cellular mislocalization and prolonged activation of Cdk5. This deregualted Cdk5 induces cytoskeletal disruption and apopotosis. To examine whether inhibition of the calapin-medaited conversion of p35 to p25 can delay the disease progression of ALS, we generate a double transgenic mice in which ALS-linked SOD1 mutant was expressed in a Cdk5 deficient background. The phenotype of these mice provide a direct evidence that the pathogenesis of motor neuron disease in the mutant SOD1 mice is independent of the Cdk5 activation by p35 or p25.